1. Field of the Invention
The present invention relates generally to optical fiber transmission systems, and particularly to a transmission systems designed such that they include compensation for dispersion across their length.
2. Technical Background
Dispersion compensation techniques in telecommunications systems or lines have been used successfully. A technique useful in lines already installed is one in which total dispersion (also called chromatic dispersion) is compensated for by an appropriately designed dispersion compensating fiber. For example, some systems include dispersion compensating fibers formed into a dispersion compensating module that can be inserted into the line at an access point, such as an end of the line. Such fibers typically have negative dispersion and may also have negative dispersion slope at 1550 nm. Several examples of dispersion compensating fibers may be found in U.S. Pat. Nos. 6,546,178; 6,445,864; U.S. Pat. No. 5,999,679; U.S. Pat. No. 5,361,319; U.S. Ser. No. 2002/0102084; U.S. Ser. No. 2002/0154877; WO 01/59496; WO 00/67053; and WO 01/73486.
Certain transmission systems have included micro-structured optical fiber to compensate for buildup of dispersion in the system. EP 1 148 360 discloses one such system including a microstructure fiber 10 (See FIG. 3) having a core region 12 surrounded by inner cladding 14 and outer cladding 16. The inner cladding 14 includes a plurality of features 14.1 (such as capillary air holes) that serve to lower the refractive index of the inner cladding. The micro-structured optical fiber 10 has a positive dispersion and is optically coupled to a negative dispersion single mode fiber having relatively small negative dispersion. The micro-structured optical fiber is used to compensate for the accumulated dispersion in the negative dispersion single mode fiber.
Another dispersion compensation scheme involves the use of both positive and negative dispersion fibers in the cables of the line. Each cable may contain both positive and negative total dispersion waveguide fibers, or the link or span can be formed using cables having only positive dispersion together with cables having only negative dispersion. This compensation scheme avoids the drawbacks associated with the dispersion compensation module, namely that the length of dispersion compensating fiber does not contribute to the span length, but necessarily complicates the installation of the system. The complication being that the dispersion sign and length of a particular cable or of the fibers in the cable must be identified during installation.
More recently, an alternative dispersion compensation technique has been developed wherein a dispersion compensating fiber is provided in the span having a total dispersion and a total dispersion slope which effectively mirrors that of the transmission fiber. That is, a ratio of total dispersion divided by total dispersion slope, kappa, has the same value for the transmission fiber and the dispersion compensating fiber. This fiber type is disclosed and discussed in U.S. Ser. No. 2002/0028051 incorporated herein by reference in its entirety. For the telecommunications system in which mirror fiber is used, the dispersion compensation is said to be perfect in that the end to end accumulated dispersion of a span including a transmission fiber and the dispersion compensating fiber is zero across the entire wavelength range of operation. The result of such a configuration is that signals in the fiber traverse significant span lengths in which the total dispersion is zero or near zero.
Other systems that include dispersion and slope compensation may be found in WO 02/099,483; U.S. Ser. No. 2003/0059186; and U.S. Ser. No. 2003/0091309. Multiple fiber solutions including at least three fibers arranged in series are taught in U.S. Pat. Nos. 5,042,906; 5,430,822; 5,781,673; 5,995,694; and U.S. patent application Ser. Nos. 2002/0102084; 2003/0039435; and 2003/0091309. However, to accomplish dispersion compensation across the entire wavelength band of interest, the amount of fiber needed to accomplish such compensation may be excessive. Moreover, such systems may suffer from excessive insertion loss.
Micro-structured optical fibers have also been employed for compensating accumulated dispersion of a transmission optical fiber in optical transmission systems. Micro-structured optical fibers are taught in U.S. Pat. Nos. 6,334,017; 6,243,522; 6,445,862; 5,802,236; 6,418,258; U.S. patent application Ser. No. 2002/0061176; and EP 1 148 360, for example.
There is, therefore, a need for optical transmission systems that provide excellent dispersion compensation over broad wavelength ranges and which compensate for dispersion with shorter lengths of optical fiber.